A fuel cell is an electrochemical device to convert chemical energy of fuel gas such as hydrogen gas and oxidizer gas containing oxygen electrochemically into electric energy. A typical fuel cell has an electrolyte membrane in contact with an anode and a cathode on either side. Fuel gas is continuously fed to the anode and the oxidizer gas is continuously fed to the cathode. The electrochemical reactions take place at the electrodes to produce an electric current through the electrolyte membrane, while supplying a complementally electric current to a load.
In a fuel cell system, the portion of the energy held by fuel which cannot be extracted as power is converted into heat. If the temperature of the fuel cell thereof is raised above an allowable temperature by this heat, the performance of the fuel cell deteriorates quickly. Accordingly, in the fuel cell system, a cooling system is provided to release heat produced in power generation thereof to the outside of the system through cooling water using a heat exchanger such as a radiator or the like.
In a proton exchange membrane fuel cell, the upper limit of the operating temperature is mainly determined from the upper temperature limit of a solid polymer electrolyte membrane. The fuel cell stack needs to be cooled so that this upper limit of the operating temperature is not exceeded.
Japanese Patent Application Laid-open Publication No. 05(2003)-074477 discloses a technology in which the power extracted from the fuel cell is limited to reduce the amount of heat generated in the fuel cell when the cooling capacity of the system is insufficient in relation to the amount of generated heat. In this technology, when the ambient temperature is too high to sufficiently release heat from a cooling device, the output of the fuel cell is limited according to the ambient temperature, thereby preventing the fuel cell from deteriorating due to excessively high temperature.